Interaction of Sequential Leaf Senescence ofPoa pratensisand Pathogenesis byDrechslera sorokinianaas Influenced by Postemergent Herbicides

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Abstract
The influence of 4 chlorophenoxy (2,4-D, 2,4,5-T, 2,4,5-TP [2,4,5-trichlorophenoxy propionic acid], MCPP) [2-(2-methyl-4-chlorophenoxy)propionic acid] and 1 benzoic acid (dicamba) postemergent herbicides on pathogenesis by D. sorokiniana on progressively older leaves of P. pratensis was determined. Disease increased on each successively older leaf of untreated control plants and a direct relationship was established between increasing leaf senescence and pathogenesis. The soil-drench application of 2,4-D and the spray and soil-drench application of 2,4,5-T, MCPP and dicamba increased the level of disease on leaves of all ages above that of the controls and on each older leaf of the plants in the respective treatments. Spray-applied 2,4-D had little influence on pathogenesis, and spray-applied 2,4,5-TP decreased disease. Extensive chlorosis and straw-colored blighting was associated with pathogenesis on the 2 oldest leaves of shoots exposed to 2,4,5-T, MCPP and dicamba and was suggestive of premature leaf senescence. Little chlorosis and blighting occurred on the 2 oldest leaves of infected plants not exposed to the herbicides and no chlorosis occurred on uninoculated controls. It was hypothesized that the increase in pathogenesis by D. sorokiniana on progressively older leaves of plants exposed to auxinlike herbicides is the function of a host-pathogen-herbicide interaction that enhances the rate of sequential leaf senescence.